Most importantly, this commit translates ZZIP_ENOENT to
std::system_error(ENOENT) so IsFileNotFound() returns true and
find_stream_art() can suppress the log line.

Apparently, Icecast defaults to ISO-8859-1 for MP3:

 http://icecast.org/docs/icecast-2.4.0/config-file.html#mountsettings

This change forces Icecast to UTF-8 without having to configure it in
Icecast's configuration file.

Fixes yet another build failure (which however only affects the 0.22
branch).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/666

Fixes another build failure.

Fixes build failure in StickerCommands.cxx.

This is the correct thing to do for ports from which a signal ultimately
"originates", such as is the case with mpd.

Commit 13208bf5 added range support to
the `move` command, but applied the wrong offset to the `to` variable.
When the source range is before the current song, and the song thus
gets decremented by the range size, then the final destination offset
must also be decremented by the range size.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/663

This is the last missing piece for https://github.com/MusicPlayerDaemon/MPD/issues/662

Commit 29f78b18 continued.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/662

Without requesting the property, "good" WebDAV servers would not send
it, and so MPD could never recognize a directory, failing the database
update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Add a runtime sanity check to avoid the assertion failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/660

Apparently, libmpcdec sets gain/peak variables to zero if they are not
present.  This clashes with our formula and results in bogus values
which cause noise during playback.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/640

The formula did not consider the channel count.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/639